Security tags are widely used to be affixed to merchandises to protect the merchandises from unauthorized removal. An acoustic magnetic tag, as one type of the typical security tag, can include a housing having an opening, a resonant sheet, a biasing magnetic sheet, a separator plate, and a sealing film. In operation, the resonant sheet, the separator plate, and the biasing magnetic sheet are assembled into the housing in sequence, and then the opening of the housing is sealed by the sealing film.